Method for depositing pyrolytic graphite



May 6 1969 M. TURKAT ET AL 3,442,617

METHOD FOR DEPOSITING PYROLYTIC GRAPHITE 25 To VACUUM PUMP (Emma/s7) 14HEATING ELEMENT QEQUJRED (31000 0e HIGHER)- THICKNESS B Up OPENINGS F02flow 12 T o Exrmvsr INJECTOBS DJLl/ENT {E a Tsnpzmrma Conmofwo) PLUSHALOGEN HYDEOCARBON (ELEMENT/1L o2 May 6 1969 METHOD Filed June 28, 1967H pgocmeaozv M. TURKAT ET AL 3,442,617

FOR DEPOSITING PYROLYTIC GRAPHITE Sheet of 2 To VAC UUM PUMP (Exmwz)HEATING ELEMENT (2100 '0 0R Hausa) Oxzmzae Sven As.- CARBON MoNoxzuE,CARBON DzoxmE,

5 1mm PL vs DILUENT (11mm 645) United States Patent 3,442,617 METHOD FORDEPOSITIN G PYROLYTIC GRAPHITE Michael Turkat, New York, and William A.Robba, Shoreham, N.Y., assignors to Chas. Pfizer & Co., Inc., New York,N.Y., a corporation of Delaware Continuation-impart of application Ser.No. 345,487,

Feb. 17, 1964. This application June 28, 1967, Ser.

Int. Cl. C01b 31/00 US. Cl. 23209.1 2 Claims ABSTRACT THE DISCLOSURECROSS-REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of our earlier copending application, Ser. No.345,487 filed on Feb. 17, 1964, now abandoned.

The present invention comprises a novel method for depositing pyrolyticgraphite.

Pyrolytic graphite is pure polycrystalline graphite which hasconventionally been deposited upon a substrate or mandrel by a carbonbearing vapor at temperatures at about or in excess of 2000 C. Thecrystalline structure of pyrolytic graphite is evidenced by an unusuallyhigh degree of preferred orientation, the crystal lattice forming alamination of layer planes ordinarily substantially parallel to eachother and, where a mandrel is used, substantially parallel to thesurface of the mandrel. Its physical properties are such that it may beutilized wherever environmental extremes of high temperature stress anderosion are experienced, as in missile nose cones and rocket thrustnozzles.

Heretofore the method for manufacturing pyrolytic graphite has been tocrack a hydrocarbon bearing gas such as methane, propane or benzene atreduced pressures, at temperatures approximating and above ,2000" C.Rates of deposition thus realized have ranged between 5 and 40 mils ofpyrolytic graphite per hour. The product usually obtained in the upperregion of such range of deposition ratesi.e., 20 to 40 mils per hourisgenerally of poor quality, having disordered crystalline growth andhaving nodule formations resulting from the creation of soot particlescaused by gas phase nucleation.

Better quality material has been obtained with chemically pure methaneat lower regions of such range of deposition rates-i.e., 6 to 10 milsper hour-in the production of flat plates of pyrolytic graphite, but incurved pieces, the long time required to produce thick portions causesgrowth stresses which cause excessive delamination and cracking oninternal surfaces of the curved pieces, whether they be produced on maleor female mandrels. For this reason, it has heretofore been impossibleto produce massive deposits of pyrolytic graphite without excessivegrowth stresses and resultant cracks. General limitations on depositthickness have been about 1% inches for plates and less than A inch forcoated or curved structures for radii of curvature less than 6 3,442,617Patented May 6, 1969 ice inches. Heretofore it has been extremelydifficult, if not impossible, to produce thick complex shapes onspecially designed mandrels due to growth and anisotropy stresses whichcombine to delaminate "and crack the product.

Accordingly, the present invention provides methods for the manufactureand formation of pyrolytic graphite having rates of deposition greatlyin excess of those heretofore attainable.

The present invent-ion also provides methods for the manufacture andformation of pyrolytic graphite of quality superior to that heretoforeattainable, such material having very high purity, well ordered anduniform crystalline growth, having negligible or no nodule formationsand having extremely little soot.

The present invention further provides a method for the manufacture andformation of massive bulk deposits of pyrolytic graphite, such depositshaving no structural defects such as excessive growth stresses orinternal surface cracks. The first bulk deposit of pyrolytic graphitemade according to the present invention had a diameter of approximately7 inches with much larger and more massive deposits being foreseen.

The present invention still further provides methods for the manufactureand formation of pyrolytic graphite which, although possibly containingdelaminations at room temperature, maintains its structural integrity athigh temperatures and under severe environmental conditions due to theconformity and tightness of its planes.

In the drawing:

FIGURE 1 is a partially cut-away representation of an electric furnaceshowing a method of introduction of reactants into the depositionchamber.

FIGURE 2 is a phantom representation of the furnace of FIGURE 1, showinga second method of introduction of reactants into the depositionchamber.

Referring to the drawing, an electric furnace 10 has heating elementscapable of heating the interior of furnace 10 to 2100 C. or higher. Theinterior of furnace 10 contains a deposition chamber 12 and an exhaustchamber 14, the latter disposed generally above the former. Twoinjectors 16, 18 extend from outside furnace 10 to deposition chamber12, both of said injectors ending in said deposition chamber 12 andhaving, respectively, nozzles 20, 22 at such ends. Injectors 16 and 18are normally made from stainless steel and, because of the hightemperatures experienced within furnace 10*, are water cooled to preventmelting. Other materials, such as graphite, are also used. An exhausttube 28 extends from exhaust chamber 14 to a vacuum pump outside furnace10.

The reactions utilized in the present invention involve the oxidation orhalogenation of a hydrocarbon alone or in combination with thermalreduction of the hydrocarbon.

The typical formula for thermal reduction of a hydrocarbon is:

(A) hydrocarbon+heat carbon+hydrogen The typical formulae for oxidationof a hydrocarbon are:

(B) hydrocarbon+oxygen carbon+hydrogen oxide (C) hydrogen-l-oxygencompound carbon+hydrogen oxide The typical formulae for halogenation ofa hydrocarbon are:

(D) hydrocarbon-l-halogen carbon-l-hydrogen halide (E)hydrocarbon-l-halogen compound carbon+hydro gen halide One aspect ofthis invention is the thermal reduction of acetylene under specificprocess conditions. Although the use of acetylene as a hydrocarbonsource gas for the production of pyrolytic graphite has been publishedmany times, the results achieved have never been very satisfactory or atmost have been only equivalent to the results achieved by using otherhydrocarbons such as methane. For the most part, any attempts toduplicate the production of pyrolytic graphite with acetylene by usingprocess conditions similar to those of methane have resulted in aninferior product. For this reason the standard gas used for themanufacture of pyrolytic graphite has been methane or, if economy ismore important than high purity, natural gas has been used.

We have discovered that, by using acetylene at certain pressures andwith certain flows, it is possible to obtain a rapid deposition ratewhich produces pyrolytic graphite of superior quality. Specifically, inthe temperature range 1900 C. to 2300 C. with a flow of acetylene suchas to maintain a furnace pressure of between 1 mm. and 6 mm. Hg absolutepressure mm. Hg being full vacuum), a deposition rate of between 17 and60 mils per hour is obtained. The deposition rate is strongly dependentupon pressure, volume, hydrocarbon concentration, temperature and area.High pressures (e.g., 4.5 mm.), flow 7 1.p.m., in small volumes (e.g.,0.25 cubic foot) at high temperature (e.g., 2100 C.) can producedeposition rates of 50 to 60 mils per hour, but these conditions areclose to sooting and so it is safer to operate at 3.5 mm., flow 1.p.m.,in 0.25 cubic foot at 2100 C. wherein the deposition rate will beapproximately 25 to 30 mils per hour and higher quality materialresults.

Numerous test runs are recorded which substantiate these results. Anextremely important consequence of this rapid deposition rate is thereduced effect of growth stresses which permit those who use thisinvention to make thicker and heavier pieces of pyrolytic graphitewithout internal cracking, though delamination of layers is stillpresent. Growth stresses result when pyrolytic graphite depositedinitially is held at its deposition temperature while additional layersare depositing. The initial layers expand while soaking at suchtemperature, thus stressing the material that is put down later andaggravating the stress problem upon cool down. Thus the shorter the timerequired for deposition the more uniform will be the material deposited,and there will be less stress upon cool down. Anisotropy stresses willstill be present, but these are reduced by applying another techniquepertaining to this invention, that is, the use of temperatureprogramming to reduce residual stresses in the material. Thisprogramming relieves stresses by counteracting mandrel stress when usinginternal or so-called female-type mandrels. This is done by increasingthe temperature during the run. This increase also relieves growthstresses. A typical case using acetylene would start depositing at 1950C. and raise the temperature 25 C. every 3 hours until, in 24 hours, thetemperature is 2150. At a deposition rate of 25 mils per hour average,0.6 inch of pyrolytic graphite have been produced. Normally with methaneit would take from 60 to 120 hours to produce the same thickness. It hasalso been found in this invention that slow cool down is important,since it helps distribute remaining residual stresses more uniformly.

The inclusion of diluents, hydrocarbon or non-carbon containing, in theacetylene stream is useful in achieving optimal deposition rates,greater evenness of deposition and in producing a higher quality ofpyrolytic graphite. When inert, non-carbon containing diluents are usedto improve the evenness of deposition, the average deposition rate, withthe diluent, remains substantially equivalent to the average depositionrate, without diluents, at any given acetylene partial pressure. Thetotal pressure, of course, will be increased where diluents are usedtogether with acetylene at a constant partial pressure of acetylene.Suitable inert diluents include nitrogen, argon or helium. Hydrogen,which may affect the reaction, may also be used. Where diluents of anytype are used the acetylene partial pressure should be held at 1 to 6mm. Hg.

It has further been found that where hydrocarbon diluents such asmethane are used, the deposition rate with methane as a diluent ishigher than the deposition rate with non-hydrocarbons as diluents, atthe same partial pressure of acetylene. Consequently, the use of methaneor another hydrocarbon as a diluent enables the production of more evendeposits of pyrolytic graphite at deposition rates somewhat higher thanthat which are achievable with the use of inert diluents at the samepartial pressure of acetylene. A convenient and economical source ofmethane for this purpose is natural gas.

Other hydrocarbons may also be used as carbon containing diluents. Theseinclude, but are not limited to, propane, butane and benzene. Generally,hydrocarbons containing from one to six carbon atoms may be used.

Another aspect of this invention is the use of an activator inconjunction with a hydrocarbon. The hydrocarbons normally used includeacetylene, benzene, propane and butane.

The addition of a second gas (oxygen, oxygen compound, halogen orhalogen compound) to the hydrocarbon increases or accelerates the rateof deposition and, when appropriate compounds are used, suppliesadditional carbon for deposition. The activator or reaction agent usedmust not cause gas phase reduction of the hydrocarbon to carbon, butrather must accelerate the removal of hydrogen from the depositionsurface 27 by combining with the hydrogen where the reaction is takingplace.

Certain of the reactions employed may occur at room temperature. It istherefore undesirable to allow the reaction to take place outsidedeposition chamber 12 as soot will form, detracting from both thequality and quantity of the pyrolytic graphite. It is mainly for thisreason that injectors 16 and 18 extend into deposition chamber 12 andprevent contact between the reactants until the last moment. An exampleof such a reaction is Example 3 below.

Additional reasons for using multiple injectors include more uniformdistribution of the gases in the working region, the reduction of gasphase nucleation and a directable stream impinging upon the depositionsurface.

For reactions which occur too slowly, even at the high temperaturesprevalent in deposition chamber 12, a mixing chamber 30 is used beforethe reactants reach deposition chamber 12. Such reactions typically arehydrocarbon oxidations. With mixing chamber 30 only one injector 32 isrequired. Other physical and mechanical aspects of a. furnace 10a usingsuch a mixing chamber 30 are similar to those of furnace 10.

Examples of reactions are:

Example 1 is the thermal reduction, or cracking, of the hydrocarbon. InExample 2 a halogen has been added as an activator. In Example 3 acarbon halogen compound has been added to act as an activator, and tosupply additional carbon for deposition. Examples 4 and 5 are additionaltypical reactions.

Examples of the results obtained are as follows.

Example I One injector introduced C H at 4.0 liters per minute, and theother injector introduced the inert gas argon at 0.1 liter per minute.Temperature was 2lO0 C., furnace pressure was 3.5 mm. Hg absolute, andarea of deposition surface approximately 10 square inches. Thedeposition rate was found to be 20 mils per hour.

Example II Conditions of Example I above were duplicated. C H was againintroduced at 4.0 liters per minute. Chlorine replaced the argon and wassimilarly introduced at 0.1 liter per minute. The deposition rate was 40mils per hour.

Example III Conditions of Example I above were duplicated. Chlorine wasintroduced at a constant 0.1 liter per minute. C H- was introducedinitially at 7.5 liters per minute and reduced by 0.5 liter per minuteevery two hours after the start. The average deposition rate was 65 milsper hour.

Example IV One injector introduced acetylene into the furnace at 4.0liters per minute and the other injector introduced methane at 2.0liters per minute. The furnace tempera ture was 2100 C., and the totalfurnace pressure was 4.5 mm. Hg absolute. The deposition rate was foundto be 25 mils per hour with an overall deposited carbon yield of 90%based on carbon initially contained in the acetylene.

The pyrolytic graphite obtained as described in the above runs was ofgood quality, having a density of 2.2 gm./cm.3 and low nodule content.

There is normally injected with the reactants a diluent gas such ashydrogen or a hydrogen, inert gas combination.

What is claimed is:

1. A deposition method of making high quality pyrolytic graphite,comprising the steps of: cracking acetylene under a pressure of about2.5-5 mm. Hg absolute, at a temperature above 2000 C., in the presenceof an activating amount of a halogenating agent selected from the groupconsisting of halogen and those compounds of halogen which combine withhydrogen to remove it at the deposition surface but do not cause gasphase reduction of the acetylene; removing the gaseous product of thecracking; and collecting the solid pyrolytic graphite product thereof ona deposition surface; all three steps occurring concurrently and at thepoint of contact with the dep osition surface.

2. The method of claim 1 wherein the activating agent is chlorine.

References Cited UNITED STATES PATENTS 1,238,734 9/1917 Averill23--209.l X 3,107,180 10/1963 Diefendorf ll7226 3,138,435 6/1964Diefendorf 23-209.l

EDWARD J. MEROS, Primary Examiner.

US. Cl. X.R. 117-46, 226

